I noticed in your screenshot that in the context toolbar, wet edges is ticked. Wet edges causes your brush strokes to be kind of like watercolor which has transparency. If you untick wet edges, I think your problem will be solved.

Hi,
The problem is when the iStock file is opened in AD, the image part of the file is not over the work area. If you zoom out a bit you should be able to see the image and use the move tool to reposition it in the work area.
Edit: Same thing with the Shutterstock file.